Total hip arthroplasty is often used to restore function to a diseased or injured hip joint. Positions and directions relative to the hip joint may be described in terms of proximal being nearer the hip joint, distal being further from the hip joint, anterior being nearer the front of the body, posterior being nearer the back of the body, medial being nearer the centerline of the body, and lateral being further from the center line of the body. In total hip arthroplasty, the surfaces of the femur and pelvis are cut away and replaced with substitute implants. In a typical case, the implants include a hip stem component, a femoral head component, an acetabular component and bone cement.
The femoral bone is prepared by reaming the femoral canal down into the bone along an axis from a proximal position near the hip joint at the upper end of the femur toward a distal position nearer the knee joint at the lower end of the femur. The pelvis is prepared by reaming the acetabulum. The implants may be placed directly in contact with the prepared bone surfaces for bony fixation of the implant. Alternatively, bone cement may be introduced into the prepared canal and acetabulum so that it hardens around and locks the components in place.
The hip stem component includes a stem portion extending down into the intramedullary canal of the femur and a neck portion extending away from the femur to support the femoral head component. Some cemented hip stem designs rely on a tapered and highly polished exterior surface for use with cement to permit subsidence of the stem into the cement when the cement undergoes load induced deformation in use. By subsiding with the deforming cement, the implant maintains intimate contact with the cement.
Occasionally an undesirable upward force can occur that may withdraw a femoral component from the femoral canal. For example, if the femoral head dislocates from the acetabular component, a surgeon may attempt to reposition them by manipulating the patient's leg in a process known as a closed reduction. Sometimes, during this procedure, the femoral head can catch on the edge of the acetabular component causing the femoral implant to be lifted up such that surgical intervention is required.
Furthermore, for a cemented implant, positioning the femoral component in the correct orientation within the cement is important for proper biomechanical functioning and long term stability of the implants. Proper placement results in a uniform and strong cement mantle around the component. Proper placement further results in appropriate loading of the implants. Femoral components, especially collarless ones, are sometimes placed at the wrong angle in the mediolateral direction. The typical situation is a varus placement in which the angle between the neck and femoral axis is too shallow.